A LASER is a source of very intense, monochromatic and unidirectional beam of light These properties of a LASER light can be exploited to measure long distances. The distance of moon from the earth has been already determined very precisely using a LASER as a source of light. A LASER light beamed at the moon takes 2.56 s to return after reflection at the moon’s surface. How much is the radius of lunar orbit around the earth ?

Time taken by a laser beam to return after reflection at the moon’s surface (T) = 2.56 s

Time taken by a laser beam to reach the moon

t=T/2 = 2.56 /2 = 1.28 s

Speed of laser beam = Speed of light © = 3 x ${{10}^{8}}$ m/s

Distance of the moon from the earth (D) = c xt

= 3 x${{10}^{8}}$ x 1.28

=3.84 x ${{10}^{8}}$

=3.84 x$ {{10}^{5}}$ km

.’. Radius of lunar orbit around the earth = 3.84 x $ {{10}^{5}}$